16.92 inches, to be exact.
There are necks out there with 18"~20" radii; anything 18+ inches is just about flat, if you ask me. I recently made a new fretboard for my RG7621 and I radiused it at 16". You can't tell the difference between the 430mm radius (again, 16.92") fretboard on my UV777BK from the new fretboard I made for the 7621. The difference in height at the center of the fretboard between the two radii is 0.0024". That's twenty-four ten-thousandths
of one inch - I verified this by drawing up my plans for the fretboard in CADD and measured. Ain't much.
The flatter fretboards on Ibanez guitars
are very comfortable to me...